154 Herbert Eugene Walter 



such a kind that it better insures the existence of the individual, 

 or of the race" (T. H. Morgan '03, p. 1). 



It is evident that the generally negative character of the reac- 

 tions of planarians to light indicates a tendency on the part of 

 these worms to reduce as much as possible the amount of light 

 stimulation received or to avoid it altogether. The rigor effects 

 of excessive stimulation furnish evidence also that light is a factor 

 in a planarian's environment which it finds unavoidable and 

 unwelcome and to which it is adapted only in a negative fashion. 

 In fact the vague distinction separating "lower" from "higher" 

 animals consists largely in the ability of higher animals to assume 

 an active aggressive rather than a passive defensive relation toward 

 the factors making up their environment. For example, the 

 evolution in animals of the visual organs, which in the planarians 

 is only inceptive, enlarges the possible range of photic responses 

 until light becomes an essential factor in an animal's environment, 

 contributing largely to its welfare by enabling it to see its food, to 

 avoid its enemies and to select its mates. It is plain that light 

 plays no such important part in the activities of planarians, for, 

 as has already been pointed out, light per se is not essential to pla- 

 narians, since they are known to live successfully in dark caves. 

 Moreover, so far as known, light does not influence the regenera- 

 tive or reproductive processes of planarians in any way whatso- 

 ever. The formation of pigment may perhaps be regarded as an 

 adaptation to light conditions, inasmuch as animals possessing 

 pigment are thereby shielded to a certain degree from excessive 

 stimulation. 



With reference to activities connected with nutrition and repro- 

 duction, planarians are not dependent upon light stimulation. 

 They are otherwise equipped, since they doubtless find their food 

 by chemotactic means and avoid whatever enemies they may have, 

 not aggressively nor actively by retreating from visible foes but rather 

 in a passive way by remaining concealed from enemies that might 

 see them. They have no organs of defense but survive by escap- 

 ing attention. In this sense their negative phototaxis may be 

 regarded as of protective value and consequently adaptive. 



Furthermore, the geographical distribution of fresh water pla- 



